Photoelectric two axis positioning system

ABSTRACT

A member is capable of being positioned at precise rectangular coordinate points by providing a circular hole in the member. A light source is positioned on one side of the member and a photocell on the other side to receive light passing through the hole along a given axis corresponding precisely to the coordinate points. A slot is provided between the photocell and hole and extends radially from the given axis beyond the boundary of the hole. By revolving this slot about the said given axis, the photocell will then receive a constant amount of light through the slot for every revolved position only if the hole in the member is exactly centered on the given axis. Any off-centering of the hole from this given axis results in a variable amount of light passing to the photocell from the slot. The resulting variable electrical signal derived from the photocell is then used to actuate servo-motors to move the member in a proper direction to minimize the variations in the signal thereby effecting exact centering of the hole and thus the member at the precise coordinate points.

ZDUfLUDOH AU LDC) CA Marantette et al. 1 Feb. 13, 1973 [54]PHOTOELECTRIC TWO AXIS [57] ABSTRACT POSITIONING SYSTEM A member iscapable of being positioned at precise [76] Inventors: William F.Marantette; Ruth B. rectangular coordinate points by providing acircular Marantette, both of 20624 Earl St., hole in the member. A lightsource is positioned on Torrance, Calif. one side of the member and aphotocell on the other I [22] Filed: g 26, 1971 s de to reeelve lightpassing through the hole along a given axis correspondmg precisely tothe coordlnate [21] Appl. No.: 175,258 points. A slot is providedbetween the photocell and hole and extends radially from the given axisbeyond the boundary of the hole. By revolving this slot about ..250/203R, 250/2836???) the said given axis, the photocell will then receive a Ii t constant amount of through the slot for every [58] held ofSearchmzSo/zoz 203 203 revolved position only if the hole in the memberis ex- 83/71 365 actly centered on the given axis. Any off-centering ofthe hole from this given axis results in a variable [56] Rderences cuedamount of light passing to the photocell from the slot. UNITED STATESPATENTS The resulting variable electrical signal derived from thephotocell is then used to actuate servo-motors to 3,293,439 l2/l966Marantette etal. ..250/203 R move the member in a proper direction tominimize 3. 3. 3/l964 A y et al l A v 250/2 the variations in the signalthereby effecting exact con- 3'230'379 Amyan e r 350/203 R tering of thehole and thus the member at the precise 2,967,247 1 H961 Torck .1...250/203 1 Cmrdimnc poinm 3,532.893 l0/l970 Marantettc et al...250/202 X 3,433,961 3/l969 Pincus ..250/203 R i 7 Claims, 5 DrawingFigures Primary ExaminerWalter Stolwein Attorney-Pastoriza & KellyPATENTEDFEBI 31913 3,716.? 1 6 Y II QUADRANT II QUADRANT 23 23 I 28 I 8QUADRANT QUADRANT 2 2? 22 I11 m QUADRANT 27 QUADRANT 11 QUADRANT HQUADRANT FIG. 2 FIG. 3 A 33 PET I QUAD I QUAD PHASE L42 PULSE GATE 6' mv1 4o PHOTO VOLTAGE OFF 1! QUAD 1: QUAD R2 PHASE CELL LIN CKT PULSE GATEc2 mv 11 I9 5e 1; 4|

CURRENT OFF n1 QUAD m QUAD R5 R6 22 29 SENSOR CKT PULSE GATE C31 lsAwTooT 35 r m QUAD mouAo R4 R7 R9 EN. PULSE GATE (AI 8 FIG. 4

VOLTAGE I n m N H m m SAWTOOTH A 0' 1 TIME CELL Tl TZT T3- T4 43 ouTPuT,r

0' /T mm 4 47 5 F V F \J PHOTOELECTRIC TWO AXIS POSITIONING SYSTEM Thisinvention relates broadly to a photoelectric two axis positioning systemparticularly useful for positioning a work in an X, Y coordinate planein accordance with given coordinate points on a master control membersuch as a template or master art work in order that automatic machiningoperations can be carried out accurately.

BACKGROUND OF THE IN VENTION In our U.S. Pat. No. 3,293,439 issued Dec.20, 1966 and entitled TWO AXIS PHOTOELECTRIC POSI- TIONING SYSTEMINCLUDING ALTERNATE AXIS ACTIVATION OF PHOTOCELL, there is described aphotoelectric means for effecting positioning of a master template atprecise rectangular coordinate points. Essentially, a photocell isprovided on one side of the master template and a light source on theother, the master template including a hole which the light passes.Off-centering of the hole changes the light pattern on the photocellgiving rise to signals which operate servomotors to move the mastertemplate in a proper direction to exactly center the hole relative tothe light path. These movements also control a circuit board or otherwork through which a hole is to be drilled at a desired coordinateposition.

While the system briefly described above operates satisfactorily, it isfound that in utilizing the holes in a master template, there is often avariation in the size or diameter of the hole with the result thatconsistent operation cannot always be realized. Further, the mastertemplate may be defective such that light passes through a noncircularopening to strike the cell and such could operate the drilling mechanismthereby drilling a fast hole. Finally, for extremely large holes in themaster template, off-centering of the hole cannot be as accuratelydetected as is desirable for precision operations.

BRIEF DESCRIPTION OF THE PRESENT INVENTION With the foregoingconsiderations in mind, the present invention contemplates an improvedphotoelectric two axis positioning system wherein far greater accuracyin precisely centering a hole relative to fixed rectangular coordinatesis realizable even though the hole itself may vary in diameter.

In accord with the invention, a member such as a master art work havinga circular hole is positioned between a light source and a photocell,the path of light being on an axis normal to the member and passingprecisely through the given coordinate points. A slot is providedbetween the photocell and the hole in the member, the slot extendingradially from the given axis beyond the boundary of the hole. With thisarrangement, it will be evident that by revolving the slot about theaxis of the light path, the photocell will receive a constant amount oflight for any revolved position of the slot only when the exact centerof the hole in the member coincides with the axis of revolution. Anyoffcentering of the hole from the axis will result in a variable amountof light passing to the cell over a complete rotation thereby enabling avariable signal to be derived from the cell. This signal may be employedto operate servo-motors to move the member or master art work in adirection to minimize the variation in the signal and thereby assure anexact centering of the hole in the member at the given coordinatepoints.

The invention is preferably utilized with a circuit board drillingmachine wherein the board is connected to the master art work formovement therewith. Suitable points on the circuit board to be drilledcan then be precisely positioned by positioning corresponding holes inthe master art work by means of the photocell and revolving slotarrangement. When used to control the drilling of a circuit board, thesystem also contemplates safety means for preventing drilling should thehole in the art work not be circular or should unwanted ambient lightstrike the cell or in some other unusual event.

BRIEF DESCRIPTION OF THE DRAWINGS A better understanding of thisinvention will be had by referring to the accompanying drawings, inwhich:

FIG. I is a schematic diagram illustrating the two axis positioningsystem of this invention as used to control the proper positioning of acircuit board through which holes are to be drilled at preciselocations;

FIG. 2 is a plan view fragmentary in form of the hole in a master artwork relative to certain components of the photoelectric system whereinthe hole is precisely centered;

FIG. 3 is a view similar to FIG. tions when the hole is off-center;

FIG. 4 is an electrical block diagram illustrating one means forutilizing generated signals for effecting an exact centering of themaster art work; and

FIG. 5 illustrates a series of wave forms useful in explaining theoperation of the system of FIG. 4.

2 illustrating condi- DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTReferring to FIG. 1 there is shown a work table 10 carrying a boardwhich might, for example, be a printed circuit board through which holesare to be drilled at various locations as by an automatic drill 12. Theboard 11 may be moved in X and Y coordinate directions as indicated bythe double headed arrows by means of a servo-motor 13 arranged to move afirst or base table I4 back and forth in the X direction and aservo-motor 15 carried on the base table I4 coupled to move the table I0back and forth in a Y direction.

Signals to the servo-motors l3 and 15 are provided from a controlcircuit designated generally by the nu merals 16 through leads I7 and 18respectively. Thus a positive signal on the lead I7 will move the table14, for example, to the right or in a positive X direction whereas anegative signal will move the table in a negative X direction or to theleft. Similarly, positive and negative signals passed to the motor 15will move the table 10 in a positive or negative Y direction. A zerovoltage signal or absence of a signal will result in nonoperation of theservo-motors thereby holding the tables and thus the board 11 in a fixedposition relative to the drill 12.

The control circuit 16 is responsive to the output of a photocell 19.This photocell includes a housing or frame structure 20 carrying a disc21 within which there is provided a slot 22. A member suchas a filmdefining a master art work 23 is disposed below the photocell and alight source 24 is positioned on the other side or beneath the member23. Means are provided to rotate the photocell and slot 22 very rapidlyas by a pulley belt 25 and drive motor 26.

The member 23 which, as stated, may comprise a master art work defines acircular hole 27 surrounded by an opaque or darkened area 28. Asindicated by the dashed line L, the circuit board 11 is connected to themaster art work 23 so that the same will move in unison. Thus operationof the servo-motors l3 and 15 to move the board 11 will also result insimultaneous movements of the master art work in X and Y directions.

As shown in FIG. 1, the master art work 23 is initially positioned sothat light from the source 24 will pass through the hole 27 and slot 22to the photocell 19. The axis of rotation of the photocell and slotstructure is designated A and is coincident with the light path from thelight source 24. This axis defines a precise coordinate point on whichit is desired to exactly center the hole 27. It will be noted that theslot 22 relative to this given axis of rotation A extends radially fromthe axis and as will be clearer as the description proceeds of a lengthto cross the boundary of the hole 27 when viewed along the axis A fromabove.

Still referring to FIG. 1, there is indicated by the block 29 a sensorpositioned adjacent to the periphery of the disc 21. This disc carries asmall metal slug 30 which will periodically pass the sensor 29 uponrotation of the housing structure about the axis A. It is not necessarythat this slug be located on the disc 21 within which the slot 22 isformed. Any rotating part of the structure could carry the slug with thesensor 29 positioned adjacent thereto to provide a signal each rotationof the photocell structure. This signal is passed through a lead 31 tothe control circuit 16. Also, there is provided suitable brush leads 32and 33 to pick up signals off suitable slip rings on the photocell l9and pass the same to the control circuit 16 as shown.

Referring now to FIG. 2, there is shown a fragmentary portion of themaster art work 23 wherein the circular hole 27 is shown preciselycentered with respect to the axis A of rotation of the photocellstructure described in FIG. 1. The slot 22 is shown projected in solidlines as it would appear when the hole is precisely centered. It will benoted that this slot extends radially from the axis A and crosses theboundary of the hole 27. The circumferentially extending series ofarrows indicate the rotation of the photocell about the axis A andsuccessive positions of the slot 22 which revolves about this axis areindicated by the dotted lines.

It will be immediately evident from FIG. 2 that the amount of lightpassing through the hole 27 and the portion of the slot 22 which is noteclipsed by the area outside the boundary of the hole is preciselyconstant for any revolved position of the slot. Such is necessarily thecase since the hole 27 is circular and the exact center of the holecoincides with the axis of revolution.

Under the above conditions, and dividing the revolving travel of theslot into four quadrants as indicated by the Roman numerals I, II, IIIand Iv, the light reaching the photocell of FIG. 1 will be the same ineach quadrant.

Referring nor to FIG. 3, there is depicted the same situation as shownin FIG. 2 except the hole 27 is offcenter from the axis A. Under theseconditions, it will be clear that more light will be passed to thephotocell while the slot is revolving through the quadrant I than whenpassing through the remaining quadrants. In other words, for anycondition in which the hole is offcentered, the amount of light reachingthe cell over a complete revolution of the slot will vary. In theparticular example depicted in FIG. 3, the hole is off-center along theX axis to the left with the result that there is more light receivedwhen the slot passes through the first quadrant than is the case whenthe slot passes through the third quadrant. The amount of light passingto the cell when the slot moves through the second and fourth quadrantsis the same.

In essence, the photoelectric cell 19 will provide a signal which isconstant so long as the amount of light reaching the cell is constantover each revolution of the cell and which is variable if the lightreaching the cell over each revolution is variable. Thus, whenever thehole 27 is off-center, there will be provided a variable signal from thephotocell to the control circuit 16 of FIG. 1. This control circuit 16together with the servomotors l3 and 15 function in response to suchvariable signal to move the table 10 and connected master art work 23along the X or Y coordinates in a proper direction tending to minimizethe variation in the electrical signal such that when the hole 27 isprecisely centered there will no longer be a variable signal so that theservo-motors will stop and motion of the table and thus the master artwork will cease. Since the master art work holes define precisecoordinate positions corresponding to points on the board 11 to bedrilled, after the board has stopped in a given position, acorresponding hole may be drilled in the board 11 as indicated at 34 inFIG. 1. After drilling, the motors 13 and 15 are energized to move theboard 11 and thus the master art work generally to position the nexthole such as 27' in the master art work 23 and a corresponding point 34'on the board 11 to be next drilled. Initial positioning of the next hole27' on the master art work between the light source 24 and photocell 19will permit light to pass though the hole to the photocell and thenthrough the control system described, exact centering of this hole willbe effected. Drilling can then take place to drill the next hole in theboard 11.

In FIG. 1 there are shown leads 35 and 36 from the control circuit block16 to safety means depicted by the block 37 connected to the drill asshown at 38. Essentially, and as will become clearer as the descriptionproceeds, certain signals on the lines 35 and 36 will act on the circuitblock 37 to prevent operation of the drill. Such signals are generatedon the lines 35 and 36 in response to certain conditions which mightotherwise cause operation of the drill resulting thereby in a falsedrill hole.

Referring now to FIG. 4, one circuit control system which could beutilized in the block 16 for carrying out the various operationsdescribed is shown. In FIG. 4, components corresponding to thosedescribed in FIG. 1 are designated by the same numerals. Thus asschematically indicated, the photocell 19 is positioned along the axis Afor receiving light passing up through the hole 27 and slot 22, the hole27 being shown as offcentered to correspond to its position illustratedin FIG. 3. The photocell 19 is preferably a silicon diode of very highimpedance and having a very fast response time. To reduce the outputimpedance so that the signal will not be lost, the cell output is firstfed into a field effect transistor through the line 33 from the slipring structure as indicated by the block FET. The block FET alsoincludes a circuit to provide a variable load resistor (transistor)which maintains the d.c. level of the photocell output near zeroregardless of the size of the master art work hole. The output from theblock FET will therefore be a constant amplitude signal so long as thehole is precisely centered as described with respect to FIGS. 2 and 3.Any off-centering of the hole will result in a variation of the signalat the output on either side of a zero voltage level.

In the particular embodiment described, the photocell and slot structureare rotated about the axis A at a high speed which is maintainedabsolutely constant; for example, five thousand revolutions per minute.The slug insert 30 on the disc 21 passes the sensor block 29 once eachrevolution and serves to change the reluctance of a magnetic circuit insuch a manner as to provide a trigger pulse each revolution. Thistrigger pulse serves a reset a sawtooth generator so that the outputfrom the sawtooth generator on output line 39 is a repetitive sawtoothpattern having a period corresponding to one revolution of the photocelland slot.

Still referring to FIG. 4, the sawtooth output on lead 39 passes to fourpulse circuits which are biased to provide an output pulse when theirinputs attain a given voltage level. These four circuits are indicatedby the blocks l quad pulse, ll quad pulse, III quad pulse, and IV quadpulse. Essentially, there will be provided four sequential output pulseseach revolution of the slot structure, the pulse from each pulse blockcomprising a stretch pulse having a duration corresponding to the timethat the slot structure is passing through the designated quadrant.

The output pulses from the various quadrant pulse generators are passedrespectively to four gating circuits designated by the blocks I quadgate, II quad gate, Ill quad gate and IV quad gate. Essentially, thesepulses open the gates for a period of time during which the slot istraveling through the corresponding quadrant.

Also feeding into the gates is the output from the FET block which isderived from the output of the photocell.

The gated signals from the quad gates pass through resistances R1, R2,R3 and R4 to charge capacitors C1, C2 C3 and C4 with a voltageproportional to the signal occuring during the particular time that theslot passes through the corresponding quadrant. The voltages across thecapacitors C1 and C2 corresponding to the signals developed while theslot is passing through the first and second quadrants are inverted asby phase inverter l and phase inverter II. The outputs from these phaseinverters connect through line 40 and 41 to the ends of split resistancenetworks R5, R6 and R7, R8 respectively. The other ends of theseresistance networks connect to the capacitors C3 and C4. Center tappoints from these resistance networks constitute output signals passedto the lines 17 and 18 of FIG. I for the X and Y servo-motorsrespectively.

Referring to FIG. 5, the foregoing circuit operations will be betterunderstood. Thus, there is indicated in the top wave form, the outputfrom the sawtooth generator on the line 39 passing to the various quadpulse circuits. As mentioned, the quad pulse circuits are biased togenerate an output pulse for certain voltage levels occuring at the timeT1, T2, T3 and T4 which correspond to the points in the time that therevolving slot starts to enter each of the quadrants I, II, III and IV.

Below the sawtooth there is depicted a wave form 43 which corresponds tothe output of the photocell after passing through the FET circuit block.It will be noted that the signal 43 varies with time which results fromthe situation depicted in FIG. 3 wherein the hole 27 is off-center alongthe X axis. It will be evident that when the hole is exactly centered onthe axis of revolution of the slot as shown in FIG. 2, the output signal43 would be constant; that is, there will be no variations in thissignal.

The gates essentially pass the average value of the signal 43 occuringin each of the quadrants to the capacitors C1 through C4 so thatvoltages are developed on these capacitors of negative and positivevalues or zero value as the case may be indicated by the levels 44, 45,46 and 47 in FIG. 5. The negative voltage 44 on the capacitor C1 isinverted by the phase inverter 1 to a positive voltage is added throughthe resistances R5 and R6 to the positive voltage level 46 on thecapacitor C3 thereby providing a high positive output voltage on thelead 17. The signal on the second capacitor C2 which in the exampletaken is zero volts is added to the signal on the capacitor C4 which isalso zero volts or is of the same voltage as the capacitor C2 so thatafter inversion the net output on the line 18 is zero volts.

OPERATION With all of the foregoing description of the drawings in mind,the operation of the system will be evident. If it is first assumed thatthe hole 27 is precisely centered on the axis of revolution for thephotocell and slot as depicted in FIG. 2, it will be evident that theoutput signal from the photocell and from the block FET of FIG. 4 willbe the same in all of the four quadrants. As a consequence, each of thegates will pass an identical voltage to each of the storage capacitorsCl through C4 of FIG. 4. Regardless of the value of this particularsignal since it is the same on all of the capacitors, the signal on thecapacitor C1 after inversion by the phase inverter when added to thesignal on the capacitor C3 will precisely cancel this signal so thatthere will be zero output volts on the line 17 passing to the X axisservomotor 13. Similarly, when the signal on the capacitor C2 isinverted and added to the signal on the capacitor C4, since all of thesignals are of the same value the output on the line 18 will be zerovolts and thus the Y axis servo-control-motor 15 will not operate. Theservo-motors with no signals being received therein will thus hold theboard 11 and master art work in the precisely centered positionssuitable for drilling.

If the hole in the master art work is off-center as depicted in FIG. 3,for example, there will be developed a positive signal on the outputline 17 to the X axis servocontrol-motor 13 as a consequence of theaddition of the voltages as described. Since, the levels on thecapacitors C2 and C4 are the same, there will not be any signal to the Yaxis control servo 15. Operation of servo-motor 13 as a consequence ofthis positive voltage will move the table structure to the right asviewed in FIG. 1 thereby centering the hole 27. As the hole approachesit center position, the signals developed when the slot passes throughthe first and third quadrants will tend to equalize thereby graduallydiminishing the positive signal passed to the servo-motor until thissignal become zero. At this point, the motor will stop and the hole willbe precisely centered. Any deviation of the hole in the direction of theY axis will cause the Y axis servo-control-motor 15 to operate in thesame manner.

Three safeguard circuits are provided to prevent false operation of thedrill. For example, should the photocell 19 be operating in an areawhere the light to the photocell in unrestricted, the photocell currentwill exceed a given value when can be set by the current sensor blockconnecting to the line 32 from the lower slip ring shown in FIG. 4. Thisphotocell current sensor provides a signal on its output line 35 to thecontrol circuit 37 for the drill as described in FIG. I to render thedrill inoperative. The situation, as mentioned, will only obtain shouldthe photocell current exceed a given value.

A second and third safeguard are provided by monitoring the outputsignal from the FET circuit of FIG. 4. Thus a branch line 42 from theoutput of the PET is passed into a voltage limitator which may be set toprovide an output signal on line 36 to render the drill inoperativeshould the output voltage signal exceed given positive and negativelevels. For example, if no light is falling on the cell, the outputvoltage signal is such as to render the drill circuit inoperative. Onthe other hand, should the excursion of the signal; that is, theamplitude of a varying signal resulting from revolution of the slot falloutside a given set amplitude, the drill is also rendered inoperative.This latter situation might occur should a non-circular hole exist inthe master template. Thus any false drilling of holes is prevented forunusual conditions of this type.

From the foregoing description, it will thus be seen that the presentinvention provides an improved two axis positioning system in whichdisadvantages encountered in prior art devices are overcome.

What is claimed is:

l. A method of positioning a member at precise rectangular coordinatepoints comprising the steps of:

a. providing a circular hole in said member;

b. providing a light source on one side of said member and a photocellon the other side;

c. initially positioning said member such that said light source passeslight through said hole to said photocell, the path of light being on anaxis nonnal to said member and located at said precise coordinatepoints;

d. providing a slot between said photocell and hole extending radiallyfrom said axis beyond the boundary of said hole;

e. revolving said slot about said axis whereby said photocell receives aconstant amount of light only when the exact center of said hole in saidmember coincides with said axis, any off-centering of said hole fromsaid axis resulting in a variable amount of light passing to said cellto result in a variable electrical signal from said cell; and

f. moving said member in response to said variable electrical signal ina direction to decrease the variations in said signal whereby when saidhole is exactly centered on said axis, said electrical signal no longervaries and movement of said member ceases.

2. The method of claim 1, in which said member comprises a master artwork having more than one hole, the holes being precisely positionedrelative to each other, and including the step of providing a board tobe drilled at points spaced relatively to each other in the same manneras the holes in said master art work, said board being connected to saidmaster art work for movement therewith whereby said board ispositionable at precise coordinate points defined by the hole positionsin said master art work when said hole in said master art work isexactly centered.

3. A photoelectric two axis positioning system comprising, incombination:

a. a master art work having a circular hole therein;

b. a light source on one side of said art work and a photocell on theother side of said art work such that light passes through said holefrom said light source to said photocell along a given axis normal tothe plane of said art work;

c. means defining a slot positioned between said art work and photocellsuch that only light passing through said slot from said hole reachessaid photocell, said slot extending radially from said given axis andpassing beyond the boundary of said hole;

. means for revolving said slot about said given axis whereby light fromsaid slot reaching said photocell will be constant for every revolvedposition of said slot only if the exact center of said hole falls onsaid given axis, any off-centering of said hole from said given axisresulting in a variable amount of light reaching said photocell overeach complete revolution of said slot;

. means connected to said photocell to provide a signal constituting afunction of the amount of light received by said photocell at eachinstant of time, said signal being variable when said amount of light isvariable and constant when said amount of light is constant; and

f. servo-motor means for moving said master art work connected to beresponsive to variations in said signal to move said art work in adirection to minimize the variations in said signal whereby when saidart work is moved to exactly center said hole on said given axis, saidsignal becomes constant and said servo-motor stops.

4. A system according to claim 3, in which said master art work includesmore than one hole, the holes being precisely positioned relative toeach other; a board through which holes are to be drilled at pointsspaced relatively to each other in the same manner as said holes in saidmaster art work, said board being connected to said master art work formovement therewith; and a stationary drill above said board, wherebysaid drill may be operated to drill a hole in said board after exactcentering of a hole in said master art work has taken place therebypositioning said board means connected to receive said signal andresponsive to variations exceeding a given amplitude to render saiddrill inoperative.

7. A system according to claim 4, including means for blocking any lightfrom reaching said cell other than the light passing through said hole.

# i i t i

1. A method of positioning a member at precise rectangular coordinatepoints comprising the steps of: a. providing a circular hole in saidmember; b. providing a light source on one side of said member and aphotocell on the other side; c. initially positioning said member suchthat said light source passes light through said hole to said photocell,the path of light being on an axis normal to said member and located atsaid precise coordinate points; d. providing a slot between saidphotocell and hole extending radially from said axis beyond the boundaryof said hole; e. revolving said slot about said axis whereby saidphotocell receives a constant amount of light only when the exact centerof said hole in said member coincides with said axis, any off-centeringof said hole from said axis resulting in a variable amount of lightpassing to said cell to result in a variable electrical signal from saidcell; and f. moving said member in response to said variable electricalsignal in a direction to decrease the variations in said signal wherebywhen said hole is exactly centered on said axis, said electrical signalno longer varies and movement of said member ceases.
 1. A method ofpositioning a member at precise rectangular coordinate points comprisingthe steps of: a. providing a circular hole in said member; b. providinga light source on one side of said member and a photocell on the otherside; c. initially positioning said member such that said light sourcepasses light through said hole to said photocell, the path of lightbeing on an axis normal to said member and located at said precisecoordinate points; d. providing a slot between said photocell and holeextending radially from said axis beyond the boundary of said hole; e.revolving said slot about said axis whereby said photocell receives aconstant amount of light only when the exact center of said hole in saidmember coincides with said axis, any offcentering of said hole from saidaxis resulting in a variable amount of light passing to said cell toresult in a variable electrical signal from said cell; and f. movingsaid member in response to said variable electrical signal in adirection to decrease the variations in said signal whereby when saidhole is exactly centered on said axis, said electrical signal no longervaries and movement of said member ceases.
 2. The method of claim 1, inwhich said member comprises a master art work having more than one hole,the holes being precisely positioned relative to each other, andincluding the step of providing a board to be drilled at points spacedrelatively to each other in the same manner as the holes in said masterart work, said board being connected to said master art work formovement therewith whereby said board is positionable at precisecoordinate points defined by the hole positions in said master art workwhen said hole in said master art work is exactly centered.
 3. Aphotoelectric two axis positioning system comprising, in combination: a.a master art work having a circular hole therein; b. a light source onone side of said art work and a photocell on the other side of said artwork such that light passes through said hole from said light source tosaid photocell along a given axis normal to the plane of said art work;c. means defining a slot positioned between said art work and photocellsuch that only light passing through said slot from said hole reachessaid photocell, said slot extending radially from said given axis andpassing beyond the boundary of said hole; d. means for revolving saidslot about said given axis whereby light from said slot reaching saidphotocell will be constant for every revolved position of said slot onlyif the exact center of said hole falls on said given axis, anyoff-centering of said hole from said given axis resulting in a variableamount of light reaching said photocell over each complete revolution ofsaid slot; e. means connected to said photocell to provide a signalconstituting a function of the amount of light received by saidphotocell at each instant of time, said signal being variable when saidamount of light is variable and constant when said amount of light isconstant; and f. servo-motor means for moving said master art workconnected to be responsive to variations in said signal to move said artwork in a direction to minimize the variations in said signal wherebywhen said art work is moved to exactly center said hole on said givenaxis, said signal becomes constant and said servo-motor stops.
 4. Asystem according to claim 3, in which said master art work includes morethan one hole, the holes being precisely positioned relative to eachother; a board through which holes are to be drilled at points spacedrelatively to each other in tHe same manner as said holes in said masterart work, said board being connected to said master art work formovement therewith; and a stationary drill above said board, wherebysaid drill may be operated to drill a hole in said board after exactcentering of a hole in said master art work has taken place therebypositioning said board for drilling at a precise corresponding point onsaid board.
 5. A system according to claim 4, including safety meansconnected to receive said signal and responsive to said signal exceedinga given valve to render said drill inoperative.
 6. A system according toclaim 4, including safety means connected to receive said signal andresponsive to variations exceeding a given amplitude to render saiddrill inoperative.